Cigarettes boost MS risk, but not through nicotine

NEW YORK (Reuters Health) - Smoking cigarettes increases the risk of multiple sclerosis (MS), but the substance that makes cigarettes addictive, nicotine, doesn't seem to be at fault, Swedish researchers say.

While male smokers were 1.8 times as likely to have MS as non-smokers, and MS risk increased 1.4-fold for women who smoked, people who used Swedish snuff were actually less likely to have the disease, Dr. Anna K. Hedstrom of the Karolinska Institutet and her colleagues found.

Unlike the US version of snuff, with Swedish snuff, "there is no spitting involved," Hedstrom noted in an interview. Users tuck the snuff into their upper lip, and absorb large amounts of nicotine in the process.

"It's not a healthy thing, it's better than smoking, but it still has negative effects in the long run," the researcher added.

Eight out of nine studies investigating smoking and MS risk have found an increased likelihood of developing the disease among smokers, Hedstrom and her colleagues note in the journal Neurology. But only one study looked at "cumulative dose," or how long and how much people smoked, and MS risk, they add.

To investigate, Hedstrom and her team compared smoking and snuff use among 902 people diagnosed with MS and 1,855 healthy individuals drawn from the general population.

Fifty-seven percent of the MS patients reported being "ever-smokers" in the year before diagnosis, compared to 48 percent of controls at an equivalent point in time.

Overall, "ever-smokers" were 1.5 times more likely to have MS than "never-smokers," while the risk was 1.6-fold greater for current smokers and 1.4 times greater for past smokers compared with never-smokers.

But for snuff users, risk of MS was 20 percent lower than it was for people who had never used any kind of tobacco. The longer they'd used snuff, the lower their risk; the longer a person smoked, the higher their risk.

It's not clear why cigarette smoking might boost MS risk, although there are many theories, Hedstrom said. Cyanide is one of the many harmful compounds found in cigarette smoke, she added, and it's known to damage nerve tissue. Smokers' greater vulnerability to infections, which have been linked to MS risk, could also be a factor, according to Hedstrom.

As far as any potential benefit of nicotine, the researcher said, it's possible the chemical may have some protective effect on the nervous system; she noted that some research has linked nicotine use to a reduced risk of Parkinson's disease.

For now, the findings should help spur smokers to quit, Hedstrom said. In her study, she pointed out, smokers' MS risk fell sharply within five years of quitting.

Anyone who's at increased risk of MS -- for example, someone with a close relative with the disease -- should quit smoking if they do, Hedstrom says. "It would be really beneficial for them."